Recently, as semiconductor lasers for emitting blue violet laser light have been put into practical use, a Blu-ray Disc (hereinafter, referred to as the “BD”), which is an optical information recording medium (hereinafter, referred to also as the “optical disc”) having a higher density and a larger capacity than CDs (Compact Discs) and DVDs (Digital Versatile Discs) while having the same disc diameter therewith, has been put into practical use. A BD is an optical disc having a protective substrate thickness of about 0.1 mm used for performing information recording and reproduction with a blue violet laser light source of a wavelength of about 400 nm and an objective lens having a numerical aperture (NA) improved to 0.85. The “protective substrate thickness” means a thickness of a transparent layer (protective substrate) existing between a face of the optical disc on the laser light incidence side and the information recording layer.
A high density optical disc having a protective substrate thickness of about 0.6 mm also used with a blue violet laser light source of a wavelength of about 400 nm and an objective lens having a numerical aperture of 0.65 are also under active studies and development. (These optical discs usable for performing recording or reproduction with a blue violet laser light source are comprehensively referred to as the “high density optical discs”.)
Under the circumstances, an optical head compatible for performing information recording and reproduction by collecting laser light of different wavelengths to an information recording layer of each of these optical discs having different protective substrate thicknesses using one objective lens are now proposed.
For example, as an objective lens usable for an optical head for performing recording to or reproduction from high density optical discs and DVDs, an optical head as shown in FIG. 28 is reported by Patent Document 1. This optical head uses, as an objective lens 101, a complex lens including a reference lens 101a formed of a synthetic resin material or a glass material and a close contact lens 101b closely contacting the reference lens 101a and formed of an ultraviolet curable resin material.
FIG. 28 shows a face 111 of the reference lens 101a on the light source side, a close contact face 112 which is a face of the reference lens 101a on the optical disc side and also is a face of the close contact lens 101b on the light source side, and a face 113 of the close contact lens 101b on the optical disc side.
According to this conventional example, it is described that an aberration caused to the laser light of two wavelengths, i.e., red laser light and blue violet laser light, by a difference in the wavelength can be compensated for.
Patent Document 2 reports an objective lens, for an optical head, which provides good optical performances. The objective lens uses laser light of three wavelengths of about 405 nm, about 655 nm and about 785 nm, and provides a superb diffraction limited performance for optical discs having different protective substrate thicknesses.
According to this conventional example, it is described that recording to or reproduction from high density optical discs, DVDs and CDs can be performed with blue violet laser light, using an objective lens 201. As shown in FIG. 29, the objective lens 201 includes a reference lens 201a and a close contact lens 201b cemented to or bonded to a face 212 of the reference lens 201a on the optical disc side. The close contact lens 201b is in close contact or substantially close contact with the face 212 of the reference lens 201a and is formed of a material different from that of the reference lens 201a. 
The objective lenses described in the two conventional examples utilize the difference in the refractive index and dispersion between the reference lens and the close contact lens, and compensate for the aberration for two or three different wavelengths only by the refractive power of the lens. Therefore, as opposed to an objective lens using diffraction, the above-described objective lenses lose substantially no light amount and provide a high transmission efficiency for all the wavelengths.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-90477    Patent Document 2: International Publication No. 2004/053557 pamphlet